The present invention relates to a device for purifying exhaust gas by removing particulate matters contained in the exhaust gas from diesel engines.
Regulations are now becoming stringent year after year against the exhaust gases emitted from internal combustion engines, and particularly from diesel engines. In particular, it is becoming an urgent task to reduce particulate matters (hereinafter referred to as PMs) which chiefly comprise carbon. As a device for removing PMs from the exhaust gas, there has been known a diesel particulate filter (hereinafter referred to as DPF). The trend toward obligatorily furnishing diesel engine-mounted vehicles with a DPF is now becoming serious.
However, the DPFs with which diesel engine-mounted vehicles are furnished must be regenerated by burning the trapped PMs because the trapped PMs are deposited thereon due to repetitive operation of the engine. As a means of regeneration, there is known system in which the PMs are burned by heating them, using an electric heater or a burner. When a system that burns the PMs is employed, the PMs cannot be trapped while the DPF is under regeneration. Therefore, a system is used in which a plurality of DPFs are arranged in parallel in the exhaust gas passage and the trapping and burning are conducted alternately. This system, however, creates a problem in that a device large in scale is required. Further, systems which burn PMs have another problem of how to secure durability of the filter, since the PMs are burned at a high temperature.
Because of these reasons, systems which burn trapped PMs have not been widely employed.
As another means for trapping and regenerating PMs, for example, Japanese Patent No. 3012249 discloses a so-called continuous regeneration type DPF in which an oxidizing catalyst is disposed in the exhaust gas passage on the upstream side of the DPF, NO in the exhaust gas is oxidized into NO2 by the oxidizing catalyst, and the trapped PMs are continuously burned with NO2 while continuing the trapping of PMs. There has further been proposed another continuous regeneration type DPF for achieving the same operation effect, in which the oxidizing catalyst is directly carried on the DPF.
As a further continuous regeneration type DPF, Japanese Patent No. 2600492 teaches a system according to which the DPF carries an NOx occluding/reducing catalyst, and the trapped PMs are continuously burned by utilizing active oxygen that generates when the NOx is occluded and reduced.
In any one of these continuous regeneration type DPFs utilizing the action of the catalyst, however, it has been well known that the exhaust gas must have a predetermined temperature to continuously regenerate the DPF while trapping the PMs. When the engine is idling or is in a low-load operation state in particular, the exhaust gas has a low temperature and flows at a low flow rate. Therefore, the temperature drops before the exhaust gas arrives at the continuous regeneration type DPF, making it difficult to continue the regeneration operation while trapping the PMs to a sufficient degree.
As described above, the continuous regeneration type DPF for diesel engines has a problem in that the trapped PMs cannot be continuously regenerated when the temperature is too low. In order that the temperature of the exhaust gas flowing into the continuous regeneration type DPF will not become low, it can be contrived to arrange the continuous regeneration type DPF close to the engine. However, since there is only limited space in the engine compartment, it is difficult to mount, in the engine compartment, a continuous regeneration type DPF of a large capacity which is designed by taking the amount of trapping the PMs and the flow resistance of the exhaust gas into consideration. It is therefore necessary to mount the continuous regeneration type DPF at a position remote from the engine.
Further, even if the continuous regeneration type DPF of a large capacity could be disposed close to the engine, the exhaust gas becomes cool in the continuous regeneration type DPF of the large capacity if the operation condition continues in a state where the exhaust gas has a low temperature and flows at a low flow rate. Therefore, it becomes difficult to continuously and completely burn the trapped PMs, whereby the PMs locally remain unburned.
In view of the above-mentioned problems, the present applicant has proposed in Japanese Patent Application No. 2001-155894 a device for purifying exhaust gas of a diesel engine comprising a first continuous regeneration type DPF having a large capacity on the downstream side of the exhaust gas passage, a by-path by-passing a portion of the exhaust gas passage on the upstream side of the exhaust gas passage, i.e., at a position close to the engine (e.g., just behind the exhaust manifold), a second continuous regeneration type DPF of a small capacity disposed in the by-path, and a control valve in the exhaust gas passage in parallel with the by-path to open and close the passage, wherein the control valve is closed in a temperature region where the exhaust gas has a low temperature while the engine is in operation, so that the exhaust gas flows into the second continuous regeneration type DPF in the by-path.
According to the above-mentioned device for purifying exhaust gas of a diesel engine, when the temperature of the exhaust gas is low, the exhaust gas is permitted to flow into the second continuous regeneration type DPF which is disposed immediately after the engine and has a capacity smaller than that of the first continuous regeneration type DPF that has a large capacity to cope with the case when the exhaust gas is emitted at a large flow rate, solving the problem in that the exhaust gas is cooled in the exhaust gas passage and in the first DPF.
Here, the continuous regeneration type DPF has a characteristic feature in that the PMs are continuously oxidized and burned to regenerate the DPF while trapping the PMs, and, hence, the PMs do not accumulate in the DPF. It is, therefore, important that the PMs be oxidized and burned in amounts larger than the amounts of PMs trapped by the DPF. However, when the efficiency for oxidizing and burning the PMs changes depending upon the temperature conditions and the like of the exhaust gas, and as a result the amount of the trapped PMs becomes greater than the amount of the PMs that can be oxidized and burned, the PMs start to deposit on the DPF, and the deposited PMs become difficult to be oxidized and burned easily. Particularly, in the continuous regeneration type DPF carrying the catalyst directly on the DPF, the reaction itself of the catalyst lowers. Once the PMs are deposited, therefore, the function of the DPF is not easily recovered. Further, if deposited in increased amounts, then, the PMs ignite when the temperature of the exhaust gas becomes high, whereby the temperature is quickly raised, causing the DPF to be melt-damaged. It is therefore desired that the PMs be deposited in smaller amounts on the DPF so as always to maintain the DPF in a fresh state as much as possible.
In the device for purifying exhaust gas of diesel engines taught in the above-mentioned prior application, the second continuous regeneration type DPF of a small capacity, arranged at a position close to the engine, works to trap the PMs and to continuously regenerate the DPF under the operation condition where the temperature of the exhaust gas is low, and efficiently purifies the exhaust gas. Meanwhile, in other operation conditions, the control valve is opened, the exhaust gas does not flow into the second continuous regeneration type DPF but flows, passing through the exhaust gas passage, into the first continuous regeneration type DPF where the PMs are trapped while the DPF is continuously regenerated. That is, the second continuous regeneration type DPF is not used under the above-mentioned operation conditions. Therefore, the second continuous regeneration type DPF that is not being used may be cooled down to a considerably low temperature due to the external air and the like. When the exhaust gas passage is changed over to the second continuous regeneration type DPF in this state, the oxidizing reaction of the catalyst is not sufficiently conducted, and the PMs are not satisfactorily oxidized and burned.
In a device for purifying exhaust gas of diesel engines comprising a first continuous regeneration type diesel particulate filter arranged in the exhaust gas passage of the engine, and a second continuous regeneration type diesel particulate filter arranged in the exhaust gas passage on the upstream side of the first continuous regeneration type diesel particulate filter, it is an object of the present invention to constitute the above device so as to allow the second continuous regeneration type DPF to be always maintained in a fresh state where no PMs are deposited and to enable the oxidizing reaction of the catalyst and the continuous regeneration of the DPF to be quickly executed without problem when the exhaust gas passage is changed over to the second continuous regeneration type DPF.
In order to accomplish the above-mentioned object according to the present invention, there is provided a device for purifying exhaust gas of diesel engines having a continuous regeneration type diesel particulate filter arranged in the exhaust gas passage of the engine, which comprises a first continuous regeneration type diesel particulate filter arranged in the exhaust gas passage of the engine, a second continuous regeneration type diesel particulate filter having a capacity smaller than the capacity of the first continuous regeneration type diesel particulate filter and disposed in the exhaust gas passage on the upstream side of the first continuous regeneration type diesel particulate filter, a by-path arranged to surround the outer peripheral portion of the second continuous regeneration type diesel particulate filter, a control valve for opening and closing the by-path, an exhaust gas temperature region-detecting means for detecting the exhaust gas temperature region of the engine, and a control means for so controlling the control valve as to close the by-path when the exhaust gas temperature region of the engine detected by the exhaust gas temperature region-detecting means is in a low temperature region lower than a predetermined temperature.